An ideal amplifier increases the amplitude of the input signal without adding any distortion or noise thereto. Such ideal amplifier also has minimal NF (noise figure), optimum desired bandwidth and perfect linearity among other favorable factors. However, ideal performance cannot be achieved in practice and therefore the real-life amplifiers tend to introduce artifacts to the signal during the amplification process.
LNAs are often used as a first amplification stage in systems, e.g. (wireless) communications systems, where relatively weak input signals shall be amplified with minimum degradation of signal quality so that the subsequent stages can be implemented with less stringent requirements.
The LNAs are typically constructed as circuits comprising as few transistors as possible to minimize the noise added to the amplified signal. In addition to purely internal noise sources also interference from other elements can negatively affect the LNA performance, this is especially the case if the LNA is integrated on a common die with the other entities, which is often the case.
An LNA can be constructed by utilizing a plurality of different configurations including a resistively matched LNA 102, a feedback LNA 104, a common-base LNA 106 and an inductively-degenerated LNA 108 as visualized in FIG. 1 using bipolar transistors (˜bipolar junction transistor, BJT). The inductively degenerated LNAs have been found particularly advantageous as providing low NF with no severe drawbacks.
A simplified example of an inductively degenerated LNA is shown in FIGS. 2A and 2B.
With reference to FIG. 2A and first considering the more generic sketch with reference numeral 202, the LNA includes a MOSFET (Metal Oxide Semiconductor Field-Effect Transistor) transistor 204, a gate-source capacitor 206 and a source inductance 208. In addition, a bonding pad 203, i.e. a metallization area on the surface of a die, to accommodate a bonding wire and couple it to the transistor input is shown in the figure. At high frequencies the parasitic capacitance of the bonding pad 203 decreases the LNA performance. In addition, the parasitics of the pad 203 do not decrease when the transistor area is reduced. This is because, they depend heavily on the pad size and oxide thickness between the lower pad metal and substrate (or a ground shield). The size of the pad 203 cannot be radically decreased since it depends on the width of the bonding wire and the accuracy of the bonding machine. Thus, it is important that the effect of the bonding pad 203 in the LNA input is minimized.
A more detailed structural view of the bonding pad 203 is presented next to the reference numeral 210. Pad 203 internals are specifically highlighted by encompassing them with a broken line in the figure. The bonding pad 203 comprises a parasitic capacitance 214 to the ground and two ESD (electrical static discharge) protection diodes 218. When the pad 203 is connected to the LNA input port, its parasitic capacitance 214 and a substrate resistance 216 can significantly lower the LNA performance. The LNA input matching may be poor and part of the signal leaks to the ground. As a result, the LNA gain reduces and the NF increases. In addition, the substrate resistor 216 is itself an additional noise source degrading the LNA NF. Inductor 212 represents the bonding wire.
With reference to FIG. 2B, more modern processes may produce a slightly different pad. This pad further comprises an additional ground shield 224 (˜ground plane) below the upper metal layer(s) 222 and on top of the substrate 226. Such shield 224 is connected to either of the supply rails, whereupon the equivalent circuit does not include substrate resistance 216 thus removing one noisy component from the LNA input; the noise caused by the resistance 216 is shunted to the ground through the shield 224. However, there still exists parasitic capacitance at the LNA input. This capacitance can be taken into account in circuit design, for example, by altering the impedance level at the LNA input. Thus, the effect of this capacitor can be slightly decreased. However, the effect of pad capacitance cannot be completely removed. As a result, the LNA performance degrades as described earlier. In addition, if this capacitance limits the maximum operation frequency of the LNA, no feasible solutions exist to widen it.